1.2 Contact details of resource persons and institutions involved in the assessment and documentation of the Technology

Name of project which facilitated the documentation/ evaluation of the Technology (if relevant)

OPtimal strategies to retAIN and re-use water and nutrients in small agricultural catchments across different soil-climatic regions in Europe (OPTAIN)

Name of the institution(s) which facilitated the documentation/ evaluation of the Technology (if relevant)

Helmholtz Centre for Environmental Research (UFZ) - Germany

1.3 Conditions regarding the use of data documented through WOCAT

The compiler and key resource person(s) accept the conditions regarding the use of data documented through WOCAT:

Ja

1.4 Declaration on sustainability of the described Technology

Is the Technology described here problematic with regard to land degradation, so that it cannot be declared a sustainable land management technology?

Nee

2.1 Short description of the Technology

Definition of the Technology:

Detention ponds are artificially excavated basins that collect stormwater runoff and eroded sediment from the upstream catchment. The water stored in these ponds is slowly released into a water body or it infiltrates into the groundwater – or both. Their primary functions are flood control, erosion control and water quality improvement.

2.2 Detailed description of the Technology

Description:

This documentation focuses on a detention basin constructed to hold surface runoff at the lowest point of an agricultural catchment. A well-designed detention pond is an effective flood control and landscape management measure. It stores surface runoff and releases it through controlled flow, infiltration, or evaporation (or a combination), and reduces nutrient inputs to the adjacent water body by filtering out sediment and nutrients through particle deposition, or nutrient uptake by plants. On average, water treatment in a detention pond can remove about 50-60% of suspended solids, 30-35% of total phosphorus and total nitrogen, and 25-65% of metals. Ponds also provide habitat for terrestrial, riparian and aquatic species. In addition to their ecological impact, they have an aesthetic and cultural value to society, for example, as a demonstration of effective sustainable water management.
The detention basin described here was constructed by excavating a new depression and ditch system. It consists of a 450 m long inlet channel, a 250 m² pond, and an overflow channel that diverts excess water downstream to the nearest river (Schwarzer Schöps). However, there are no outlet structures for draining water that is captured in the pond (apart from the overflow). All the collected water seeps into the ground and soil particles settle. As a general rule, the size of such a pond should be 3-7% of the upstream catchment and should be able to hold at least the precipitation of a 1 in 30 year rainfall event. However, as the infiltration of the collected water is rather slow, the efficiency is reduced for successive events and overflowing is common. The depth should be between 1.2 m and 2.0 m. A deeper pond can lead to stratification and anoxic conditions, while a shallower pond may cause algal blooms and high biological activity in the summer. Water stored in the pond should remain for at least 20 days to ensure biological treatment.
The construction of the detention pond in this documentation was initiated as a compensation and replacement measure for the construction of a new road in 2016. Accordingly, the road construction authority, the landscape conservation association and a farmer were involved in the implementation.
As usual with such measures there are drawbacks as well as positive impact for landowners. The measure consumes a lot of land, which automatically results in yield losses. It is expensive to build and not easy to construct - and requires the help of an engineering firm while also being time-consuming to maintain. Maintenance is required to sustain the benefits of the technology. For a pond such as that documented here, the following maintenance activities are necessary: during the first three years, maintenance includes mowing the surrounding green area and the dried out pond and channel three times per year. After the fourth year, maintenance includes (a) an annual function check, (b) a check for pest infestation every two years, and (c) suppression of excess woody growth every eight to ten years. Twice a year (mid-June and late August), the grass around the pond and in the intake channel is mowed and the clippings are removed. Shrubs and woody plants are maintained, especially by pruning woody plants (every 8 to 10 years) and coppicing sections of shrubs (every 10 to 15 years). In the long term, it may be necessary to remove the deposited sediments in order to maintain the retention capacity.

2.3 Photos of the Technology

2.5 Country/ region/ locations where the Technology has been applied and which are covered by this assessment

2.6 Date of implementation

If precise year is not known, indicate approximate date:

• 10-50 years ago

2.7 Introduction of the Technology

• Compensation measure

Comments (type of project, etc.):

The construction of the detention pond was initiated as a compensation measure for the construction for the road construction project "S70 OU Reichenbach 2. BA".

3.1 Main purpose(s) of the Technology

• reduce, prevent, restore land degradation
• conserve ecosystem
• protect a watershed/ downstream areas – in combination with other Technologies
• reduce risk of disasters
• create beneficial social impact

3.2 Current land use type(s) where the Technology is applied

Land use mixed within the same land unit:

Nee

3.3 Has land use changed due to the implementation of the Technology?

Has land use changed due to the implementation of the Technology?

• No (Continue with question 3.4)

3.4 Water supply

Water supply for the land on which the Technology is applied:

• rainfed

3.5 SLM group to which the Technology belongs

• water harvesting
• ecosystem-based disaster risk reduction

3.6 SLM measures comprising the Technology

3.7 Main types of land degradation addressed by the Technology

3.8 Prevention, reduction, or restoration of land degradation

Specify the goal of the Technology with regard to land degradation:

• reduce land degradation
• restore/ rehabilitate severely degraded land

4.1 Technical drawing of the Technology

4.2 General information regarding the calculation of inputs and costs

Specify how costs and inputs were calculated:

• per Technology unit

other/ national currency (specify):

€

If relevant, indicate exchange rate from USD to local currency (e.g. 1 USD = 79.9 Brazilian Real): 1 USD =:

0.91

4.3 Establishment activities

	Activity	Timing (season)
1.	remove topsoil
2.	profile water body according to site plan (Standing water 1.5 m deep, ditch 0.5-1.5m deep)
3.	installing of inlet channel
4.	installing erosion protection on embankments
5.	cover the topsoil / terrain profiling
6.	planting of shrubs & aquatic plants along the shoreline
7.	ditch connection to Schwarzer-Schöps
8.	reprofiling ditches (90m)
9.	desilting of drainage ditch to Schwarzer-Schöps
10.	subsequent installation of a spillway (beak structure: 'Schnabelbauwerk') and willow sticks as an erosion control measure for the ditch connection to Schwarzer-Schöps.

4.4 Costs and inputs needed for establishment

If you are unable to break down the costs in the table above, give an estimation of the total costs of establishing the Technology:

25250.0

If land user bore less than 100% of costs, indicate who covered the remaining costs:

The costs were covered by the road construction authority (compensation measure for the construction of a new road)

Comments:

based on estimated average costs (nwrm.eu)

4.5 Maintenance/ recurrent activities

	Activity	Timing/ frequency
1.	completion and development maintenance (mowing of the green area and the water-free water body)	first three years: three times per year
2.	functional check	after the fourth year: every year
3.	check for pest infestation	after the fourth year : every other year
4.	check to suppress unwanted woody growth	after the fourth year: every 8 to 10 years
5.	grass around the pond and in the intake channel is mowed and the clippings are removed	after the fourth year: twice a year (mid-June and late August)
6.	pruning woody plants	every 8 to 10 years
7.	putting sections of shrubs on stock	every 10 to 15 years

4.6 Costs and inputs needed for maintenance/ recurrent activities (per year)

If you are unable to break down the costs in the table above, give an estimation of the total costs of maintaining the Technology:

618.0

If land user bore less than 100% of costs, indicate who covered the remaining costs:

The costs are covered by the road construction authority (compensation measure for the construction of a new road)

Comments:

estimated average costs (nwrm.eu)

4.7 Most important factors affecting the costs

Describe the most determinate factors affecting the costs:

From the government perspective: A budget must be set aside annually for necessary maintenance to ensure functional efficiency.

5.1 Climate

5.2 Topography

Indicate if the Technology is specifically applied in:

• concave situations

5.3 Soils

If available, attach full soil description or specify the available information, e.g. soil type, soil PH/ acidity, Cation Exchange Capacity, nitrogen, salinity etc.

Luvisol (German soil classification system: Parabraunerde)

5.4 Water availability and quality

Is water salinity a problem?

Nee

Is flooding of the area occurring?

Nee

5.5 Biodiversity

5.6 Characteristics of land users applying the Technology

5.7 Average area of land used by land users applying the Technology

5.8 Land ownership, land use rights, and water use rights

Land ownership:

• state

Land use rights:

• individual

Water use rights:

• communal (organized)

Are land use rights based on a traditional legal system?

Nee

5.9 Access to services and infrastructure

6.1 On-site impacts the Technology has shown

Socio-economic impacts

Water availability and quality

Income and costs

Ecological impacts

Water cycle/ runoff

Soil

Biodiversity: vegetation, animals

Climate and disaster risk reduction

Specify assessment of on-site impacts (measurements):

The assessment is based on expert judgements. It is not based on on-site measurements.

6.2 Off-site impacts the Technology has shown

Specify assessment of off-site impacts (measurements):

The assessment is based on the expertise of the compilers and not on measurements in the case study.

6.3 Exposure and sensitivity of the Technology to gradual climate change and climate-related extremes/ disasters (as perceived by land users)

Gradual climate change

Gradual climate change

	Season	increase or decrease	How does the Technology cope with it?
annual rainfall		decrease	well

Climate-related extremes (disasters)

Meteorological disasters

	How does the Technology cope with it?
local rainstorm	very well

Climatological disasters

	How does the Technology cope with it?
heatwave	well

Comments:

In past summers there was little or no water left in the pond.

6.4 Cost-benefit analysis

How do the benefits compare with the establishment costs (from land users’ perspective)?

How do the benefits compare with the maintenance/ recurrent costs (from land users' perspective)?

6.5 Adoption of the Technology

• single cases/ experimental

6.6 Adaptation

Has the Technology been modified recently to adapt to changing conditions?

Nee

6.7 Strengths/ advantages/ opportunities of the Technology

Strengths/ advantages/ opportunities in the land user’s view
temporary water-bearing habitat
biotopic network
buffer effect

Strengths/ advantages/ opportunities in the compiler’s or other key resource person’s view
effective flood control and landscape management measure
Stores surface runoff, slows it down, and releases it incrementally through controlled runoff, infiltration, or evaporation. Reduces nutrient inputs to adjacent water bodies.
aesthetic and cultural value for the society

6.8 Weaknesses/ disadvantages/ risks of the Technology and ways of overcoming them

Weaknesses/ disadvantages/ risks in the land user’s view	How can they be overcome?
Danger of siltation and succession if not maintained regularly and professionally.
Difficult maintenance due to inaccessible terrain.

Weaknesses/ disadvantages/ risks in the compiler’s or other key resource person’s view	How can they be overcome?
Consumes a lot of land, which means a loss of yield.
expensive to build and not easy to construct
requires the help of an engineering firm
time-consuming to maintain

7.1 Methods/ sources of information

7.3 Links to relevant online information

Title/ description:

Hunt et al. (2020): Plant Selection for Infiltrating Wet Ponds in North Carolina

URL:

https://content.ces.ncsu.edu/plant-selection-for-infiltrating-wet-ponds-in-north-carolina

Title/ description:

nwrm.eu: Individual NWRM, retention pond

URL:

http://nwrm.eu/index.php/measure/retention-ponds